In general, the generated gas vapor in the fuel tank should be discharged to the outside as the generated gas vapor increases the pressure in the fuel tank and leads to an explosion.
In order to discharge the generated gas vapor, a vent apparatus is installed in the fuel tank of the automobile. The vent apparatus, such as a fuel cap, prevents fuel leakage in the event of a sudden shock to the automobile. Such a fuel cap is disclosed in U.S. Pat. No. 4,685,584.
However, the conventional fuel cap has disadvantages in that the amount of the generated gas vapor to be discharged is limited by a time constraint. Therefore, the fuel cap cannot discharge all the generated gas vapor when the pressure of the fuel gas in the fuel tank is suddenly increased.
Further, when the fuel is smeared on a ball utilized for opening and closing a valve body, the ball adheres to the vent opening part due to the viscous property of the fuel so that the exhausted gas cannot be discharged properly through the vent opening on the rivet.